1.1Physical Chemistry

Atomic Structure — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

10 min read 22 parts Lesson 1 of 13

📚 Oxford AQA International A Level Chemistry Hub / Topical Revision

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Lesson summary

Complete PDF with figures and original page layoutPage 1 Oxford AQA International Your notes A Level (IAL) Chemistry Atomic Structure Contents Fundamental Particles Mass Number & Isotopes Electron Configuration Ionisation Energy Pa…

Part 1 of 10The structure of the atom

The structure of the atom

Atoms

Atoms the smallest parts of an element that can take part in chemical reactions, and mostly empty space around a tiny dense nucleus

Nucleus

Nucleus it contains protons and neutrons, called nucleons, and has an overall positive charge

Electrons

Electrons negatively charged electrons are found in orbitals in the space around the nucleus

Model

Model understanding of atomic structure has evolved with evidence from experiments

Part 2 of 10Subatomic particles

Relative charges
- actual charges are too small for coulombs, so they are compared with each other

Particle	Relative charge	Relative mass
Proton	+1	1
Neutron	0	1
Electron	-1	1/1836

Subatomic particles

Actual charge

Actual charge an electron has -1.602 × 10^-19 C and a proton has +1.602 × 10^-19 C

Neutral atoms

Neutral atoms the number of protons equals the number of electrons

Ions

Ions an atom that loses electrons forms a positive ion, and one that gains electrons forms a negative ion

Part 3 of 10Atomic number and mass number

1Atomic (proton) number, Z

Atomic (proton) number, Z the number of protons in the nucleus, equal to the number of electrons in an atom, and unique to each element

2Chemical change

Chemical change electrons can be lost, gained or shared, but the proton number never changes

3Mass (nucleon) number, A

Mass (nucleon) number, A the total number of protons and neutrons

4Location

Location both numbers are on the Periodic Table

Part 4 of 10Calculating particles in atoms and ions

Calculating particles in atoms and ions

Protons

Protons equal to the atomic number

Neutrons

Neutrons mass number minus atomic number

Electrons in an atom

Electrons in an atom equal to the atomic number

Electrons in an ion

Electrons in an ion atomic number adjusted by the charge, because a 2+ ion has lost two electrons and a 1- ion has gained one

Species	Protons	Neutrons	Electrons
Mg	12	12	12
O^2-	8	8	10
Al³⁺	13	14	10

Part 5 of 10Isotopes

Isotopes = atoms of the same element with the same number of protons and electrons but different numbers of neutrons

Notation

Notation the symbol or name then the mass number, such as carbon-12 and carbon-14 with 6 and 8 neutrons

Hydrogen

Hydrogen its isotopes have the same protons but different neutrons

Part 6 of 10Time of flight mass spectrometry: purpose and stages

1Purpose

Purpose to find the relative atomic mass of an element from the abundance and mass of its isotopes, to identify it,
- and to find relative molecular mass

2Common principle

Common principle the sample is ionised, accelerated, and the ions are separated by their charge-to-mass ratio

3TOF stages

TOF stages ionisation, acceleration, ion drift and detection

Part 7 of 10Time of flight: ionisation and acceleration

Time of flight: ionisation and acceleration

Ionisation

The sample is dissolved in a volatile solvent and injected through a fine hollow needle to make a mist.
The needle is on a high-voltage supply, so particles lose electrons: X(g) → X⁺(g) + e^-.
Most ions have a 1+ charge because removing further electrons is difficult.
The solvent evaporates, leaving only positive ions.

Acceleration

Positive ions are attracted to a negatively charged plate and accelerate in an electric field.
All ions gain the same kinetic energy, so their speed depends on mass.
Lighter ions move faster and heavier ions move slower.

Part 8 of 10Time of flight: ion drift and detection

Ion drift

The ions pass through a hole in the plate into a flight tube.
The time of flight depends on velocity.

Detection

Ions hit a negatively charged detector plate and gain an electron, producing a current.
The size of the current is proportional to the abundance of the ions.
Lighter ions arrive first and heavier ions arrive later.
A computer converts the signal into the mass spectrum.

Key point

Key point all ions have the same kinetic energy, so separation depends only on mass

Part 9 of 10Interpreting mass spectra

Relative abundance

Relative abundance the proportion of one isotope in the natural mixture, such as Cl-35 at 75% and Cl-37 at 25%

Peaks

Peaks their heights show abundance, and the horizontal axis is m/z, which equals the mass number for 1+ ions

Boron

Boron two peaks show two isotopes, B-10 at 19.9% and B-11 at 80.1%

Interpreting mass spectra

Multiply each isotope’s abundance by its m/z.

Add the products.

Divide by 100.

Interpreting mass spectra

Result

A_r of boron = ÷ 100 = 10.8
- (19.9 × 10 + 80.1 × 11)

Types

Types low resolution mass spectrometry gives relative atomic mass, and high resolution gives relative molecular mass of a compound

Part 10 of 10Electron energy levels

Electronic configuration

Electronic configuration the arrangement of electrons in energy levels (shells) around the nucleus

Principal quantum number, n

Principal quantum number, n it numbers the shells from 1 to 4, and the lower the n the closer to the nucleus and the lower the energy

Shell (n)	Maximum electrons
1	2
2	8
3	18
4	32

Spacing

Spacing energy levels get closer together further from the nucleus, with an overlap between the third and fourth levels

Sub-shells

Sub-shells shells divide into sub-shells s, p, d
- (and f for elements with more than 57 electrons)

Energy order

Energy order s < p < d < f

Orbital

Orbital a volume of space with a high probability of finding an electron

Content by level

Content by level the first level has one s orbital, and the second level has an s and p orbitals

Key terms

Term	Meaning
Nucleon	A proton or a neutron in the nucleus.
Isotopes	Atoms of the same element with different numbers of neutrons.
Relative isotopic abundance	The proportion of an isotope in a natural mixture.
m/z	The mass-to-charge ratio of an ion.
Principal quantum number	The number n that labels an electron shell.
Orbital	A region of space where an electron is likely to be found.

Avoid these mistakes

Saying the electron has a mass of 1 — its relative mass is 1/1836.

Changing the proton number to form an ion — ions form by changing electrons only.

Saying all ions have the same speed in TOF — they have the same kinetic energy but different speeds.

Forgetting to divide by 100 in relative atomic mass — divide the weighted sum by 100.

Last updated: 18 October 2026